1. FIELD OF THE INVENTION PA1 2. DESCRIPTION OF THE PRIOR ART
The present invention relates to detection circuitry.
Detection circuitry is now used in a wide variety of applications. For example, lift or elevator doors are provided with detection circuitry to prevent the doors closing when the presence of an obstruction is sensed in the doorways.
In automatic hand driers, detection circuitry is used to sense the approach of hands to be dried and in response thereto activates the supply of hot air.
In automatic toilets, detection circuitry is used to monitor the departure of a user from the toilet seat and, after a determined delay, the circuitry activates a flushing mechanism. The same circuitry may be used to initiate a ventilator fan in response to the approach of a user to the toilet seat.
Such circuitry may incorporate infrared transmitters and detectors. The transmitters and detectors are advantageously semiconductor transmitters and detectors, but are preferably in the form of diodes for inexpensive of manufacture.
When such diodes are used as detectors, they are generally connected in series with a resistor across a pair of power supply rails. These diodes are positioned to receive light, either directly or by reflection from a light emitting diode.
The environment in which such diodes are required to operate quite often means that the detecting diodes are subject to sunlight, which is many times more powerful than the light output of the light emitting diode. Therefore, in order to prevent the diode from saturating, the value of the series connected resistor needs to be relatively low (for example 200 ohmns). As a result, the variation in output signal level from the detector diode is relatively low, and the signal-to-noise ratio is also low.
This means in practice that the detection circuitry is generally limited to applications where the range over which detection is required is relatively low.
British patent specification 1,524,564 describes a light curtain apparatus for detecting the presence of an object in a detection zone.
A row of light generators face a row of light receptors to provide a row of beams traversing the area to be monitored.
Each light generator is energized in succession, and each light receptor is enabled in synchronism with its corresponding light generator so that only one light generator and one light receptor are active at any one time.
Thus, at any one time only one light receptor and one light generator are active.
The specification does mention in passing that two or more light receptors can correspond to one light generator and that a single light receptor can correspond to two or more light generators, but there is no disclosure as to how these are connected in a circuit.
The present invention is advantageous over the prior art in that it provides an array of light receptors and generators which are connected in a circuit so that their effects are cumulative and so that the effect of environmental lighting is substantially eliminated.
It is an object of the invention to provide an improved detection circuitry.